The magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based magnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size and magnetic exchange decoupling between the grains, play key roles in controlling the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters as the PO and grain size of the cobalt-based magnetic alloy. The PO of the various materials forming the layers on the disk is not necessarily an exclusive orientation which may found in the material, but is merely the dominant orientation. When the Cr underlayer is deposited at elevated temperature on a NiP-coated AlMg substrate a [100] preferred orientation (PO) is usually formed. This PO promotes the epitaxial growth of [11{overscore (2)}0] PO of the hcp cobalt (Co) alloy, thereby, improving the in-plane magnetic performance of the disk for longitudinal recording. The [11{overscore (2)}0] PO refers to a film of hexagonal structure whose (11{overscore (2)}0) planes are predominantly parallel to the surface of the film. Likewise the [10{overscore (1)}0] PO refers to a film of hexagonal structure whose (10{overscore (1)}0) planes are predominantly parallel to the surface of the film. Since nucleation and growth of Cr or Cr alloy underlayers on glass and most non-metallic substrates differ significantly from those on NiP-coated AlMg substrates, media fabricated on glass substrates often have larger noise compared with those made on NiP-coated AlMg substrates under identical deposition conditions. The trend toward higher rotation speeds and tighter mechanical tolerances is making NiP/AlMg substrates less desirable. The use of a judiciously chosen initial layer on the substrate (called the seed layer) allows the performance of alternative substrates to equal or exceed NiP/AlMg disks. The seed layer is formed between the alternative substrate and the underlayer in order to control nucleation and growth of the underlayer which in turn affects the magnetic layer. Several materials have been proposed for seed layers such as: Al, Cr, CrNi, Ti, Ni3P, MgO, Ta, C, W, Zr, AlN and NiAl on glass and non-metallic substrates. (See for example, “Seed Layer induced (002) crystallographic texture in NiAl underlayers,” Lee, et al., J. Appl. Phys. 79(8), 15 Apr. 1996, p. 4902ff). In a single magnetic layer disk, Laughlin, et al., have described use of an NiAl seed layer followed by a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [10{overscore (1)}0] texture in the magnetic layer. (“The Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media,” IEEE Trans. Magnetic. 32(5) September 1996, 3632).
The design of magnetic disks has progressed rapidly in recent years making improvements ever more difficult. In some metrics, e.g. signal-to-noise ratio (SNR), even 1 dB improvement is now quite significant. The further improvement in SNR remains as one of the major challenges in high density recording technology.